This is a patchset implementing MMU notifier callbacks based on Andrea's
earlier work. These are needed if Linux pages are referenced from something
else than tracked by the rmaps of the kernel (an external MMU).
The known immediate users are
KVM
- Establishes a refcount to the page via get_user_pages().
- External references are called spte.
- Has page tables to track pages whose refcount was elevated(?) but
no reverse maps.
GRU
- Simple additional hardware TLB (possibly covering multiple instances of
Linux)
- Needs TLB shootdown when the VM unmaps pages.
- Determines page address via follow_page (from interrupt context) but can
fall back to get_user_pages().
- No page reference possible since no page status is kept..
XPmem
- Allows use of a processes memory by remote instances of Linux.
- Provides its own reverse mappings to track remote pte.
- Established refcounts on the exported pages.
- Must sleep in order to wait for remote acks of ptes that are being
cleared.
Known issues:
- RCU quiescent periods are required on registering
notifiers to guarantee visibility to other processors.
Andrea's mmu_notifier #4 -> RFC V1
- Merge subsystem rmap based with Linux rmap based approach
- Move Linux rmap based notifiers out of macro
- Try to account for what locks are held while the notifiers are
called.
- Develop a patch sequence that separates out the different types of
hooks so that we can review their use.
- Avoid adding include to linux/mm_types.h
- Integrate RCU logic suggested by Peter.
V1->V2:
- Improve RCU support
- Use mmap_sem for mmu_notifier register / unregister
- Drop invalidate_page from COW, mm/fremap.c and mm/rmap.c since we
already have invalidate_range() callbacks there.
- Clean compile for !MMU_NOTIFIER
- Isolate filemap_xip strangeness into its own diff
- Pass a the flag to invalidate_range to indicate if a spinlock
is held.
- Add invalidate_all()
V2->V3:
- Further RCU fixes
- Fixes from Andrea to fixup aging and move invalidate_range() in
do_wp_page
and sys_remap_file_pages() after the pte clearing.
V3->V4:
- Drop locking and synchronize_rcu() on ->release since we know on release
that
we are the only executing thread. This is also true for invalidate_all()
so
we could drop off the mmu_notifier there early. Use hlist_del_init
instead
of hlist_del_rcu.
- Do the invalidation as begin/end pairs with the requirement that the
driver
holds off new references in between.
- Fixup filemap_xip.c
- Figure out a potential way in which XPmem can deal with locks that are
held.
- Robin's patches to make the mmu_notifier logic manage the
PageRmapExported bit.
- Strip cc list down a bit.
- Drop Peters new rcu list macro
- Add description to the core patch
V4->V5:
- Provide missing callouts for mremap.
- Provide missing callouts for copy_page_range.
- Reduce mm_struct space to zero if !MMU_NOTIFIER by #ifdeffing out
structure contents.
- Get rid of the invalidate_all() callback by moving ->release in place
of invalidate_all.
- Require holding mmap_sem on register/unregister instead of acquiring it
ourselves. In some contexts where we want to register/unregister we are
already holding mmap_sem.
- Split out the rmap support patch so that there is no need to apply
all patches for KVM and GRU.
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